Polyethylene terephthalate (PET) bulk film analysis using C60, Au3, and Au primary ion beams

The damage characteristics of polyethylene terephthalate (PET) have been studied under bombardment by C₆₀⁺, Au₃⁺ and Au⁺ primary ions. The observed damage cross-sections for the three ion beams are not dramatically different. The secondary ion yields however were significantly enhanced by the polyatomic primary ions where the secondary ion yield of the [M + H]⁺ is on average 5× higher for C₆₀⁺ than Au₃⁺ and 8× higher for Au₃⁺ than Au⁺. Damage accumulates under Au⁺ and Au₃⁺ bombardment while C₆₀⁺ bombardment shows a lack of damage accumulation throughout the depth profile of the PET thick film up to an ion dose of ∼1 × 10¹⁵ ions cm⁻². These properties of C₆₀⁺ bombardment suggest that the primary ion will be a useful molecular depth profiling tool.     

Kinetic energy distributions of neutral In and In2 sputtered by polyatomic ion bombardment

Kinetic energy distributions of neutral In monomers and In₂ dimers sputtered from a polycrystalline indium surface under bombardment with 5 keV/atom Au₁⁻ and Au₂⁻ projectiles have been investigated by means of laser postionization time-of-flight mass spectrometry. Results show that 5 keV Au₁ bombardment leads to results in full compliance with linear cascade sputtering theory. For polyatomic ion bombardment, we find a clear transition to a collisional spike dominated emission process. The spike contribution appears as a low-energy part in the sputtered flux which increases with increasing projectile nuclearity and energy. We show that, the velocity spectrum associated with the low-energy contribution is virtually identical for sputtered monomers and dimers. This finding has important implications with respect to the particle emission mechanism under polyatomic projectile bombardment.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Stretching the limits of static SIMS with C60

Pristine and Au-covered molecular films have been analyzed by ToF-SIMS (TRIFT™), using 15 keV Ga⁺ (FEI) and 15 keV C₆₀⁺ (Ionoptika) primary ion sources. The use of C₆₀⁺ leads to an enormous yield enhancement for gold clusters, especially when the amount of gold is low (2 nmol/cm²), i.e. a situation of relatively small nanoparticles well separated in space. It also allows us to extend significantly the traditional mass range of static SIMS. Under 15 keV C₆₀⁺ ion bombardment, a series of clusters up to a mass of about 20,000 Da (Au₁₀₀⁻: 19,700 Da) is detected. This large yield increase is attributed to the hydrocarbon matrix (low-atomic mass), because the yield increase observed for thick metallic films (Ag, Au) is much lower. The additional yield enhancement factors provided by the Au metallization procedure for organic ions (MetA-SIMS) have been measured under C₆₀⁺ bombardment. They reach a factor of 2 for the molecular ion and almost an order of magnitude for Irganox fragments such as C₄H₉⁺, C₁₅H₂₃O⁺ and C₁₆H₂₃O⁻.     

Matrix-enhanced cluster-SIMS

We have used 23 keV C₆₀⁺ projectiles in the event-by-event bombardment and detection mode to investigate the emission of the gramicidin S [M − H]⁻ ion embedded in a matrix of sinapic acid. We have observed an increase in the gramicidin S [M − H]⁻ ion of approximately eight times by controlling the ratio of gramicidin S to sinapic acid. The maximum of the gramicidin S [M − H]⁻ yield occurs at a matrix/analyte ratio of 10:1. This ratio is different from those typically used in matrix-assisted laser desorption/ionization.     

Influence of massive projectile size and energy on secondary ion yields from organic surfaces

We investigated the influence of the projectile size and energy using Au_n^q+ clusters (5 < n < 400, 1 < q < 4) impacting on a glycine target with a 19q-34q keV energy range. We show that both CN⁻ fragment and Gly⁻ molecular ion yields are equivalent for projectiles with n > 9 and increase with the energy per projectile atoms. A maximum yield of 0.5 (50%) for both CN⁻ and Gly⁻ was obtained with the Au₄₀₀⁴⁺ projectile at 136 keV total energy. For Gly⁻, the yield enhancement is linear for Au_n when n > 5. Trends for the CN⁻ fragment are different. A nonlinear yield enhancement proportional to n³ is observed for Au_n when n < 9.     

Substrate effects on the analysis of biomolecular layers using Au, Au3 and C60 bombardments

Effects of platinum silicon, graphite and PET substrates on the secondary ion yield of sub-monolayer and multilayer samples of Cyclosporin A following 20 keV Au⁺, Au₃⁺and C₆₀⁺ impacts have been investigated. The obtained results of sub-monolayer samples show that platinum enhances the yield of the pseudo-molecular ion following Au⁺ and Au₃⁺ impacts due to the high density of the substrate that enables the energy of the primary ions to be deposited near the surface. C₆₀⁺ impacts on sub-monolayer samples are less effective, but there is an enhancement on PET substrates. Impacts of 20 keV Au⁺ and Au₃⁺ are not very efficient on multilayer samples. 20 keV C₆₀⁺ impacts enhance the yields significantly, especially for the relatively high molecular weight [M+H]⁺ ion.     

The effect of C60 cluster ion beam bombardment in sputter depth profiling of organic-inorganic hybrid multiple thin films

The effects of C₆₀ cluster ion beam bombardment in sputter depth profiling of inorganic-organic hybrid multiple nm thin films were studied. The dependence of SIMS depth profiles on sputter ion species such as 500 eV Cs⁺, 10 keV C₆₀⁺, 20 keV C₆₀²⁺ and 30 keV C₆₀³⁺ was investigated to study the effect of cluster ion bombardment on depth resolution, sputtering yield, damage accumulation, and sampling depth.     

Organic SIMS with single massive gold projectile: Ion yield enhancement by silver metallization

The yield of CN from glycine rises from one CN per projectile impact to two when the sample is covered with a nominally 1 nm thick layer of silver. These yields were obtained for bombardment with Au₄₀₀⁴⁺ projectile with 136 keV impact energy. A multitude of CN-based clusters, e.g. Ag_xO_y(CN)_z are produced concurrently. As a result, a total of three CN-based secondary ions are generated per projectile impact. The exceptionally high yield for CN is of interest for biological SIMS.     

Influence of primary ion species on the secondary cluster ion emission process from SAMs of hexadecanethiol on gold

In order to investigate the secondary cluster ion emission process of organo-metallic compounds under keV ion bombardment, self-assembled monolayers (SAMs) of alkanethiols on gold are ideal model systems. In this experimental study, we focussed on the influence of the primary ion species on the emission processes of gold-alkanethiolate cluster ions from a hexadecanethiol SAM on gold. For this purpose, we carried out time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements using the following primary ion species and acceleration voltages: Ar⁺, Xe⁺, SF₅⁺ (10 kV), Bi⁺, Bi₃⁺(25 kV), Bi₃²⁺, Bi₅²⁺, Bi₇²⁺ (25 kV).It is well known that molecular ions M⁻ and gold-alkanethiolate cluster ions Au_xM_y⁻ with M = S-(CH₂)₁₅-CH₃, x − 3 ≤ y ≤ x + 1, x, y > 0, show intense peaks in negative mass spectra. We derived yields Y_SI exemplarily for the molecular ions M⁻ and the gold-hexadecanethiolate cluster ions Au_y+1M_y⁻ up to y = 8 and found an exponentially decreasing behaviour for increasing y-values for the cluster ions.In contrast to the well-known increase in secondary ion yield for molecular secondary ions when moving from lighter to heavier (e.g. Ar⁺ to Xe⁺) or from monoatomic to polyatomic (e.g. Xe⁺ to SF₅⁺) primary ions, we find a distinctly different behaviour for the secondary cluster ions. For polyatomic primary ions, there is a decrease in secondary ion yield for the gold-hexadecanethiolate clusters whereas the relative decrease of the secondary ion yield ξ_Y with increasing y remains almost constant for all investigated primary ions.     

C60 sputtering of organics: A study using TOF-SIMS, XPS and nanoindentation

Sputtering of organic materials using a C₆₀ primary ion beam has been demonstrated to produce significantly less accumulated damage compared to sputtering with monatomic and atomic-cluster ion beams. However, much about the dynamics of C₆₀ sputtering remains to be understood. We introduce data regarding the dynamics of C₆₀ sputtering by evaluating TOF-SIMS depth profiles of bulk poly(methyl methacrylate) (PMMA). Bulk PMMA provides an ideal test matrix with which to probe C₆₀ sputter dynamics because there is a region of steady-state secondary ion yield followed by irreversible signal degradation. C₆₀ sputtering of PMMA is evaluated as a function of incident ion kinetic energy using 10 keV C₆₀⁺, 20 keV C₆₀⁺ and 40 keV C₆₀⁺⁺ primary ions. Changes in PMMA chemistry, carbon accumulation and graphitization, and topography as a function of total C₆₀ ion dose at each accelerating potential is addressed.     

Yields and ionization probabilities of sputtered Inn particles under atomic and polyatomic Aum ion bombardment

The emission of neutral and charged atoms and clusters from a polycrystalline indium surface under bombardment with 5 and 10 keV Au, Au₂, Au₃ and Au₅ projectiles was investigated. Single photon laser postionization was utilized for the detection of sputtered neutral particles. Secondary ions were detected without the laser under otherwise exactly the same experimental conditions. The relative cluster yields were found to be enhanced under polyatomic projectile bombardment, more so the larger the number of atoms in the sputtered cluster. The ionization probability strongly increases with increasing cluster size, but is essentially independent of the projectile impact energy. At a fixed impact energy, the ionization probability of sputtered monomers was found to decrease with increasing number of constituent gold atoms per projectile, but there was no detectable effect for sputtered dimers and larger clusters.     

Analysis of TOF-SIMS spectra from fullerene compounds

We analyzed TOF-SIMS spectra obtained from three different size of fullerenes (C₆₀, C₇₀ and C₈₄) by using Ga⁺, Au⁺ and Au₃⁺ primary ion beams and investigated the fragmentation patterns, the enhancement of secondary ion yields and the restraint of fragmentation by using cluster primary ion beams compared with monoatomic primary ion beams. In the TOS-SIMS spectra from C₇₀ and C₈₄, it was found that a fragment ion, identified as C₆₀⁺ (m/z = 720), showed a relatively high intensity compared with that of other fragment ions related to C₂ depletion. It was also found that the Au₃⁺ bombardment caused intensity enhancement of intact molecules (C₆₀⁺, C₇₀⁺ and C₈₄⁺) and restrained the fragmentation due to C₂ depletion.     

Friction model to describe cluster bombardment

Short time molecular dynamics simulations were performed to model C₆₀ and Au₃ bombardment of an amorphous water sample in the projectile energy range of 5-120 keV. A previously proposed friction model has been applied to describe the fundamental motion of a projectile during cluster bombardment of a solid. This simple analytical model uses a definition of friction on a single particle to describe the cluster movement through a medium. Although the mathematics of the friction model vary among systems, the projectile motion and energy deposition of a single particle into the sample as well as the reactive environment created is close to that of C₆₀ bombardment.     

Kinetic electron emission due to perpendicular impact of carbon ions on tungsten surfaces

Total electron yields for perpendicular impact of C⁺ ions on W have been measured for projectile energies from 0.2 keV to 7 keV. The data are compared with the data of C⁺ bombardment of gold and graphite in order to demonstrate general trends in kinetic electron yields at low projectile velocities. The total electron yields in the studied combinations of projectiles and substrates show a similar exponential dependence Γ ∝ (v/A) exp(−A/v), where A is a constant and v is the projectile velocity.     

Using polyatomic primary ions to probe an amino acid and a nucleic base in water ice

In this study on pure water ice, we show that protonated water species [H₂O]_nH⁺ are more prevalent than (H₂O)_n⁺ ions after bombardment by Au⁺ monoatomic and Au₃⁺ and C₆₀⁺ polyatomic projectiles. This data also reveals significant differences in water cluster yields under bombardment by these three projectiles. The amino acid alanine and the nucleic base adenine in solution have been studied and have been shown to have an effect on the water cluster ion yields observed using an Au₃⁺ ion beam.     

Au-analyte adducts resulting from single massive gold cluster impacts

Utilizing Au₄₀₀⁴⁺ primary ions produces large molecular ion yields, some in excess of unity, with minimal surface damage. A surprising observation is the occurrence of Au-analyte adducts as part of the ejecta desorbed by a single Au-cluster impact. We present data that demonstrate that Au and Au-adducts as secondary ions (e.g., AuCN⁻, AuGly⁻ and AuCsI⁻) are the result of the interaction between a single primary ion, Au₄₀₀⁴⁺ and the target atoms.     

Characterization and ion-induced degradation of cross-linked poly(methyl methacrylate) studied using time of flight secondary ion mass spectrometry

In this study, a series of random copolymers of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) were prepared as surface-initiated polymer (SIP) films on silicon substrates using atom transfer radical polymerization. Positive and negative ion static time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to characterize SIP films with different MMA/EGDMA monomer ratios in an attempt to quantify their surface composition. However, matrix effects in the positive and negative ion modes led to preferential secondary ion generation from the EGDMA monomer and suppression of secondary ions characteristic of the MMA monomer, precluding accurate quantification using standard linear quantification methods. Ion-induced degradation of these films under 5 keV SF₅⁺ bombardment was also examined to determine the effect of cross-linking on the accumulation of ion-induced damage. Increasing incorporation of the EGDMA cross-linker in the SIP films decreased the sputter rate and increased the rate of damage accumulation under extended (>10¹⁴ ions/cm²) 5 keV SF₅⁺ bombardment. Comparison of the ion bombardment data with thermal degradation of cross-linked PMMA suggests that the presence of the cross-linker impedes degradation by depolymerization, resulting in ion-induced damage accumulation. The increased rate of ion-induced damage accumulation with increased cross-link density also suggests that polymers that can form cross-links during ion bombardment are less amenable to depth profiling using polyatomic primary ions.     

Sputtering of thin benzene and polystyrene overlayers by keV Ga and C60 bombardment

The mechanisms of ion-stimulated desorption of thin organic overlayers deposited on metal substrates by mono- and polyatomic projectiles are examined using molecular dynamics (MD) computer simulations. A monolayer of polystyrene tetramers (PS4) physisorbed on Ag{1 1 1} is irradiated by 15 keV Ga and C₆₀ projectiles at normal incidence. The results are compared with the data obtained for a benzene overlayer to investigate the differences in sputtering mechanisms of weakly and strongly bound organic molecules. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C₆₀, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C₆₀ and Ga projectiles. Our data indicate that chemical analysis of the very thin organic films performed by detection of sputtered neutrals will not benefit from the use of C₆₀ projectiles.     

Sputtering of amorphous ice induced by C60 and Au3 clusters

Molecular dynamics simulations were performed to study the behavior of cluster SIMS. Two predominant cluster ion beam sources, C₆₀ and Au₃, were chosen for comparison. An amorphous water ice substrate was bombarded with incident energy of 5 keV. The C₆₀ cluster was observed to shatter upon impact creating a crater of damage approximately 8 nm deep. Although Au₃ was also found to both break apart and form a damage crater, it continued along its initial trajectory causing damage roughly 10 nm deep into the sample and becoming completely imbedded. It is suggested that this difference in behavior is due to the large mass of Au relative to the substrate water molecule.